In the age of multimedia, high volume and high quality video and audio data and even high quality game software have occupied a great part of the market. These data need to be stored in a fast-accessing, low cost and high capacity storage medium, and is preferably able to efficiently make spare copies. Various recordable/rewritable optical discs and corresponding recording apparatus having the feature of making a copy of large amount of data in an inexpensive way are thus developed. An optical disc is commonly used for storing large amount of video and audio data, software, or material and configuration data in professional applications. Therefore, not only has the optical disc recording apparatus become indispensable peripheral equipment for both personal computers and laptops in today's computer industry, in the mainstream digital consumer market, optical disc recording apparatus have begun playing an important role. Users who frequently use the optical disc recording apparatus to create a copy of data into a commercial recordable/rewritable optical disc that is pre-designed with monotonous and common label side might suffer from distinguishing these recorded discs.
Conventionally, permanent markers or special pens are used to mark the recorded disc, but human's handwritings are subject to inconvenience or misunderstanding. Printed labels stuck on the non-data side of the recorded disc are another option to specify the information of the disc. The requirements on weight distribution and adhesion of the labels are critical because the uneven weight distribution would adversely affect the rotation of the disc and the fallen-off label could jam the machine.
In light of these issues, a special dye layer that can be burned to form a desired configuration is provided on the label layer of the optical disc. In this way, the label side can be provided with desired marks such as patterns or letters. Marking the label side of an optical disc is generally performed after data is written into the data side of the optical disc. The disc is taken out of the optical disc recorder, flipped to the other side and placed back into the optical disc recorder, and the optical head of the optical disc recorder then projects laser light onto the label side of the optical disc where the special dye is applied to induce a chemical reaction, thereby changing the color of the dye layer and forming a desired pattern on the label side.
Please refer to FIG. 1A which schematically shows the label side of a recordable/rewritable optical disc. The optical disc 10 includes a concentric center hole 13, an annular information area 12 and an annular reference region 11 disposed between the center hole 13 and the information area 12, which is also referred to as a label zone. The annular reference region 11, which is also referred to as a control feature zone, is previously provided with certain patterns and includes an outer ring 14 and an inner ring 16, as shown in FIG. 1B. The outer ring 14 that is not uniformly patterned is recorded with a media ID, a saw tooth and an index mark. The inner ring 16, on the other hand, is provided with a uniform pattern, i.e. alternate “dark” and “bright” spokes, for rotation control while marking the label side. Meanwhile, the saw tooth on the outer ring 14 is used for shift calibration of the optical head, and the media ID and index mark provide other reference information relating to the optical disc 10. In general, the reference information of the outer ring 14 is accessed by the optical head, while the reference information of the inner ring 16 is realized by a spoke detector.
By detecting the patterns of the outer ring 14 and the inner ring 16, the reference information recorded in the control feature zone 11 can be realized. Typically, two ways are adopted for reading reference information recorded in the control feature zone 11, i.e. an open-loop focusing control mechanism and a closed-loop focusing control mechanism. Compared with the open-loop focusing control, the closed-loop focusing control may consume less reading time. In practice, before the optical head reads the reference information recorded in the control feature zone 11 via the closed-loop focusing control mechanism, a startup procedure is performed by way of a focus servo technology to realize a so-called “S-curve”, which represents the relationship between voltages of focusing error signal FE and distances of the focusing positions on the optical disc from an object lens of the optical head. It is understood by those ordinary in the art that the voltage of the focusing error signal depends on the intensity of the laser light focused on certain spot in the control feature zone 11 of the optical disc via the object lens and then reflected by the optical disc. By modulating the S-curve, a proper focusing error gain (FE Gain) can be obtained by the optical head to execute accurate focusing control for the subsequent reading procedure.
As described above, the control feature zone 11 includes irregular saw-tooth or block patterns. Therefore, both highly reflective zones and low reflective zones are involved. Basically, the information indicated by the S-curve is relatively reliable when it is realized from the highly reflective zones. Unfortunately, it is practically probable that the S-curve is realized from the low reflective zones or the interfaces between the highly and low reflective zones due to its the short-period feature. Under this circumstance, the resulting S-curve would be defective and causes signal distortion.
FIG. 2A exemplifies a typical S-curve. As shown, the waveform of the S-curve is so intact and smooth that an accurate and reasonable FE gain can be obtained accordingly. On the other hand, if the entire or partial S-curve is undesirably realized from a low reflective zone, a destructed waveform would be rendered, as depicted in FIG. 2B. Then the resulting FE gain for subsequent focusing control could be improper.